fix: close review gaps for per-model threshold overrides (#63020)

Follow-up to the salvaged contributor commit, closing the three gaps
flagged in the sweeper review:

1. Init ordering: assign compression.model_thresholds to a selected
   plugin context engine BEFORE the initial update_model() call in
   agent_init.py, so the initial model's override applies from init
   (previously it only took effect after the first /model switch).
   Base-class ContextEngine.update_model() now snapshots the
   pre-override percent once so repeated switches fall back to the
   engine's configured threshold, not a previous model's override.
2. DEFAULT_CONFIG: add compression.model_thresholds (empty map) to
   hermes_cli/config.py — additive key, no _config_version bump.
3. Docs: document the key in
   website/docs/developer-guide/context-compression-and-caching.md
   (yaml example, parameter table, dedicated section) and update the
   plugin-boundary note in context-engine-plugin.md to state the
   explicit context-engine contract for model_thresholds.

Adds tests/run_agent/test_per_model_threshold_init_ordering.py:
plugin-engine AIAgent init regression (override applies at init,
empty map unchanged), DEFAULT_CONFIG key presence, floor interaction
on the model-switch path (override below the small-context floor is
raised to the floor; above the floor wins), and base-class config
snapshot across repeated switches. Also maps @bennybuoy in
contributors/emails/.

website/docs/developer-guide/context-engine-plugin.md

@@ -165,7 +165,7 @@ context:
   engine: "lcm"   # must match your engine's name property
 ```
 
-The `compression` config block (`compression.threshold`, `compression.protect_last_n`, etc.) is specific to the built-in `ContextCompressor`. Your engine should define its own config format if needed, reading from `config.yaml` during initialization.
+The `compression` config block (`compression.threshold`, `compression.protect_last_n`, etc.) is specific to the built-in `ContextCompressor`, with one explicit exception: `compression.model_thresholds` (per-model threshold overrides) is part of the context-engine contract. The host assigns the resolved map to `engine.model_thresholds` *before* the initial `update_model()` call, and the base-class `update_model()` applies it (longest substring match, falling back to the engine's configured threshold). Engines that override `update_model()` own their own compaction policy and may honor or ignore the map — `from agent.context_compressor import resolve_model_threshold` to reuse the same resolution logic. For everything else, your engine should define its own config format if needed, reading from `config.yaml` during initialization.
 
 ## Testing